This proposal deals with the development of new and improved radiopharmaceuticals based on steroid molecules. Steroid molecules will be labeled with short-lived radionuclides and structure activity studies of the effects of structural modifications on the kinetics of distribution, metabolism and elimination will be conducted. Such radiopharmaceuticals could form the basis of a non-invasive in vivo assay of tissue levels of specific enzymes and/or receptor sites. Three interrelated approaches will be used to design radiopharmaceuticals for the adrenal, mammary and prostate glands. The first is to test the hypothesis that suicide-substrate inhibitors can be used to produce radiopharmaceuticals which can be used to image and/or measure tissue levels of a specific enzyme. The second is to test the hypothesis that steroid analogs can be found to image specific steroid receptor sites. The third is to test the hypothesis that analogs of naturally occuring steroid hormones can be designed which are only metabolized by a certain pathway. The preparation and study of the types f steroid analogs described in this proposal will generate the information necessary to design radiopharmaceuticals for imaging and/or measuring tissue levels of specific steroid receptor sites or steroid metabolizing enzymes. The work in this proposal should lead to effective radiopharmaceuticals for imaging tumors of the adrenal, mammary and prostate glands and for the study of other diseases of those organs.